The invention relates to a gas discharge lamp for dielectrically impeded discharges, which gas discharge lamp is provided with a discharge vessel filled with a gas filling, which discharge vessel comprises at least a wall of a dielectric material and at least a wall having a surface which is at least partly transparent to visible radiation and coated with a phosphor layer, and provided with an electrode structure for a dielectrically impeded discharge, and with means for igniting and maintaining the dielectrically impeded discharge.
Gas discharge lamps for dielectrically impeded discharges are used, particularly, in office automation devices, for example color copiers and color scanners, in signal devices, for example as brake lights and direction indicator lights in automobiles, in auxiliary illumination devices, for example, the courtesy lighting of automobiles, as well as, in particular, for the background lighting of displays and display screens, for example of liquid crystal displays.
Said applications require the luminance to be both uniform throughout the length of the lamp and high. To increase the luminance, it is necessary to increase the power that is coupled into the system. However, this also causes a higher load to be imposed on, inter alia, the phosphors in the phosphor layer. The phosphors degrade more rapidly and the luminous output decreases more rapidly during operation as the power coupled in increases.
In particular gas discharge lamps for dielectrically impeded discharges comprising phosphors containing Eu2+ as the activator are adversely affected by said degradation of the phosphors. Phosphors comprising Eu2+ as the activator are disclosed in xe2x80x9cUllmans Enzyklopxc3xa4die der technischen Chemiexe2x80x9d, 16, 195 (1978), Weinheim.
Therefore, it is an object of the invention to provide a gas discharge lamp for dielectrically impeded discharges, which gas discharge lamp is provided with a discharge vessel filled with a gas filling, which discharge vessel comprises at least a wall of a dielectric material and at least a wall having a surface which is at least partly transparent to visible radiation and coated with a phosphor layer, which phosphor layer comprises a phosphor containing Eu2+ as the activator, and which gas discharge lamp is also provided with an electrode structure for a dielectrically impeded discharge, and with means for igniting and maintaining the dielectrically impeded discharge, in which discharge lamp degradation of the phosphor is reduced.
In accordance with the invention, this object is achieved by a gas discharge lamp for dielectrically impeded discharges, which gas discharge lamp is provided with a discharge vessel filled with a gas filling, which discharge vessel comprises at least a wall of a dielectric material and at least a wall having a surface which is at least partly transparent to visible radiation and coated with a phosphor layer, which phosphor layer comprises a phosphor having a host lattice, Eu2+ as the activator and a doping D selected from the group formed by Ce3+, Pr3+ and Tb3+, and provided with an electrode structure for a dielectrically impeded discharge and means for igniting and maintaining the discharge.
In a gas discharge lamp with a phosphor layer, which comprises a phosphor with Eu2+ as the activator and a doping D selected from the group formed by Ce3+, Pr3+ and Tb3+, the degradation of the phosphor is reduced and the color point of the lamp remains unchanged.
The invention is based on the recognition that the degradation of phosphors comprising Eu2+ as the activator in gas discharge lamps for dielectrically impeded discharges due to photo-oxidation of said Eu2+ is caused by the VUV radiation from the gas discharge lamp having a wavelength in the range from 100 to 200 nm, whereas excitation by means of light having a longer wavelength in the range from 200 to 400 nm does not lead to any observable loss in efficiency. It has been found that this photo-oxidation of the Eu2+ ion can be precluded by adding further dopants, such as Ce3+, Pr3+ and Tb3+ to the phosphor, said dopants being themselves photo-oxidizable.
In accordance with a preferred embodiment of the invention, the host lattice is composed of an inorganic material selected from the group formed by oxides, sulphides, halogenides, aluminates, gallates, thiogallates, phosphates, borates and silicates.
It is particularly preferred that the phosphor is selected from the group formed by BaMgAl10O17:Eu2+,D; BaMgAl10O17:Eu2+,Mn,D; (Sr,Ba)MgAl10O17:Eu2+,D; BaMgAl16O27:Eu2+,D; BaMgAl14O23:Eu2+,D; CaAl12O19:Eu2+D; SrAl12O19;Eu2+,D; SrAl2O4:Eu2+,D; (Ca,Sr,Ba)5(PO4)3Cl:Eu2+,D; Sr5(PO4)3Cl:Eu2+,D; Ba3MgSi2O8Eu2+,D; BaF(Cl,Br):Eu2+,D and SrGa2S4:Eu2+,D.
Within the scope of the invention, it is preferred that the phosphor layer comprises BaMgAl10O17:Eu2+,D; Zn2SiO4:Mn and (Y,Gd)BO3:Eu3+.
It may alternatively preferred that the phosphor layer comprises BaMgAl10O17:Eu2+,D; (Y,Gd)BO3:Tb3+ and (Y,Gd)BO3:Eu3+.
The invention further relates to a phosphor comprising a host lattice, Eu2+ as the activator and a doping D selected from the group formed by Ce3+, Pr3+ and Tb3+. Such a phosphor can also be advantageously used for the phosphor layer of colored plasma display screens.